The present invention relates to a semiconductor device with pressure sensing elements, and more particularly to a silicon diaphragm type pressure sensor.
Unlike past mechanical pressure sensors employing a Bourdon tube, etc., the semiconductor pressure sensor (having, e.g., a silicon diaphragm) permits microminiaturization, reduced cost and enhanced performance. The silicon diaphragm type pressure sensor is typical of the semiconductor pressure sensor.
The structure of the silicon diaphragm type pressure sensor will be briefly explained. As shown in FIG. 1, a depressed portion 2 is formed by etching the rear surface (side 1B) of a silicon single-crystal substrate 1, so as to provide a membrane or thin-walled portion 3 as a diaphragm, and elongate diffused resistor layers 4 connected into a bridge are disposed in the major surface of the diaphragm 3. When a pressure difference develops in spaces over and under the diaphragm, this diaphragm is deformed in accordance with the pressure difference. Owing to the deformation of the diaphragm, the diffused resistor layers expand or shrink to change their resistances. The pressure sensor senses a pressure change by electrically detecting the change of the resistances.
FIG. 2 is a view showing the structure of a pressure sensing element which has been thought out by the inventors prior to the present invention. As illustrated in the figure, in the silicon diaphragm type pressure sensor, metal electrodes 7 made of aluminum or the like are connected through contact holes 6 to the diffused resistor layer 4 serving as the pressure sensing element.
In the structure of FIG. 2, the diffused resistor layer 4 bent in the shape of the letter U is formed in the major surface ((100) plane) of the diaphragm 3, and it is held in ohmic contact with the aluminum leads 7 through the contact holes 6. The aluminum leads 7 are extended as wiring leads on the front surface of the thick-walled semiconductor body 1 around the diaphragm 3.
The present inventors have recognized that the following problems are involved in the pressure sensor of such structure:
(1) With the U-shaped diffused resistor layer as shown in FIG. 2, the changes of the resistance caused by the deformation of the diaphragm are in a sense opposite to each other comparing the straight parts and the part extending between the straight parts and are canceled, so that the resistance change of the whole diffused resistor becomes small. This will be elucidated with reference to FIG. 4, FIGS. 5(a)-5(c) and FIGS. 6(a)-6(c).
FIG. 4 is a plan view schematically showing the silicon diaphragm portion, and FIGS. 5(a) and 6(a) are sectional views taken along lines A-A' and B-B' in FIG. 4, respectively. Assuming that the diaphragm 3 has been deformed downwards, the resistor 4a of the corner part (that is, the part extending between the two straight parts or, generally, the part extending between the parts which perform an actual pressure sensing operation) undergoes a stress as indicated by arrows in FIG. 5(a). On the other hand, the resistors 4b of the straight parts which perform an actual pressure sensing operation undergo stresses as indicated by arrows in FIG. 6(a). FIGS. 5(b) and 5(c) and FIGS. 6(b) and 6(c) are views which show in plan the changes of the shapes of the respective resistors 4a and 4b before and after the deformation of the diaphragm. As seen from FIGS. 5(b) and 5(c), the resistor 4a has the relations of W.sub.1 &gt;W.sub.2 and R.sub.1 &lt;R.sub.2, where W.sub.1 denotes the width of this resistor before the deformation of the diaphragm and R.sub.1 the resistance thereof at that time, and W.sub.2 denotes the width of this resistor after the deformation of the diaphragm and R.sub.2 the resistance thereof at the time. Meanwhile, as seen from FIGS. 6(b) and 6(c), each resistor 4b has the relations of W.sub.3 &lt;W.sub.4 and R.sub.3 &gt;R.sub.4 where W.sub.3 denotes the width of this resistor before the deformation of the diaphragm and R.sub.3 the resistance thereof at that time, and W.sub.4 denotes the width of this resistor after the deformation of the diaphragm and R.sub.4 the resistance thereof at that time.
Thus, even when the pressure change is sensed as the change of the resistance by the resistors 4b of the straight parts, the resistance change is canceled because the change of the resistance in the resistor 4a of the part extending between the two straight parts is opposite in since to that in the resistor 4b of the straight part. Therefore, the sensitivity of the pressure sensor becomes low.
(2) FIG. 3 is a sectional view taken along line A-A' in FIG. 2. With the structure shown in FIG. 3 wherein aluminum is directly connected to the resistor layer 4 on the silicon diaphragm, there is the problem that a precise pressure measurement is impossible on account of a strain which arises between the aluminum 7 and an oxide film (e.g., SiO.sub.2 film) 8 on the silicon surface. More specifically, since the difference between the coefficients of thermal expansion of the aluminum and the SiO.sub.2 film is great, the strain of the grain boundary of the aluminum stresses the SiO.sub.2 film and the surface of the silicon membrane portion directly below it, with the result that residual strains develop in the diaphragm surface (as indicated by T in FIG. 3). It has been revealed that the residual strains exert the evil effect of causing variation in the characteristics of diaphragm type pressure sensors, etc. and that they set limits to providing a pressure sensor of high precision.
(3) As shown in FIG. 2, the p.sup.- -type resistor layer 4 is unitarily formed, and hence, the resistance Ra of the part extending between the two straight parts is comparatively high. Letting Rb denote the resistance of the straight part of the resistor layer 4, and .DELTA.Rb denote the variation of the resistance Rb based on the deformation of the silicon diaphragm, the pressure sensitivity .alpha. of the pressure sensor is substantially given by the following equation: ##EQU1## As understood from Equation (1), when the resistance Ra is high, the pressure sensitivity .alpha. becomes low.
The three problems mentioned above have been found by the inventors.